Pump for loop reactor

ABSTRACT

The present invention relates the use of a pump in a loop reactor for the production of polyethylene, as well as a reactor comprising such pump and methods for producing polyolefin by means of such reactor. The pump according to the invention is characterized in that it is an axial flow impeller circulation pump, wherein the impeller comprises 6 blades and wherein the pump is fixed on a spring supported frame. Use of the pump according to the present invention allows for preparation of homogeneous polyethylene products that meet high quality standards from the complicated ethylene polymerization mixtures while at the same time being produced with low energy consumption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2011/063146, filed Jul. 29,2011, which claims priority from EP 10171364.2, filed Jul. 30, 2010.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a reactor for producing polyolefin. Thepresent invention specifically relates to the use of a pump in a loopreactor. In particular, the invention relates to the use of a pumphaving an impeller system designed for the displacement and continuouscirculation of a slurry polymerization mixture in a loop reactor.

BACKGROUND OF THE INVENTION

Polyolefin such as polyethylene (PE) is synthesized by polymerizingolefin monomer such as ethylene (CH₂═CH₂) monomers. Because it is cheap,safe, stable to most environments and easy to be processed polyethylenepolymers are useful in many applications. According to the propertiespolyethylene can be classified into several types, such as but, notlimited to, LDPE (Low Density Polyethylene), LLDPE (Linear Low DensityPolyethylene), and HDPE (High Density Polyethylene). Each type ofpolyethylene has different properties and characteristics.

Olefin polymerization such as ethylene polymerizations are frequentlycarried out in a loop reactor using ethylene monomer, liquid diluent andcatalyst, optionally one or more co-monomer(s), and hydrogen. Thepolymerization in a loop reactor is usually performed under slurryconditions, with the produced polymer usually in a form of solidparticles which are suspended in the diluent. The slurry in the reactoris circulated continuously with a pump to maintain efficient suspensionof the polymer solid particles in the liquid diluent. Polymer slurry isdischarged from the loop reactor by means of settling legs, whichoperate on a batch principle to recover the slurry. Settling in the legsis used to increase the solids concentration of the slurry finallyrecovered as product slurry. The product slurry is further dischargedthrough heated flash lines to a flash tank, where most of the diluentand unreacted monomers are flashed off and recycled.

Alternatively, the product slurry may be fed to a second loop reactorserially connected to the first loop reactor wherein a second polymerfraction may be produced. Typically, when two reactors in series areemployed in this manner, the resultant polymer product is a bimodalpolymer product, which comprises a first polymer fraction produced inthe first reactor and a second polymer fraction produced in the secondreactor, and has a bimodal molecular weight distribution.

After the polymer product is collected from the reactor and the diluentresidues are removed therefrom, the polymer product is dried, additivescan be added and finally the polymer may be extruded and pelletized.

The inventors have found that there are numerous challenges in theproduction of high-yield and high-quality polyolefin such aspolyethylene. They include adequate control of different reactionconditions such as temperature, pressure and flow rate. Effective designand maintenance of reactors are also required. The complex nature of thereaction slurry presents further complications. The slurry is forinstance affected by the quantity and quality of ingredients or thehomogeneity of the particulate polymerization catalyst dispersion whichis prone to sedimentation in the slurry. Furthermore, high energyconsumption needs to be avoided to lower production costs. In view ofthese challenges, there remains a need in the art for improved equipmentto meet the rigorous requirements for high quality polyethyleneproduction.

It is therefore the objective of the invention to provide a polyolefinpolymerization reactor, in particular a slurry polymerization loopreactor for the production of polyolefin such as polyethylene, meetinghigh quality standards, wherein additionally energy consumption isoptimized.

SUMMARY OF THE INVENTION

The inventors have surprisingly found that the above-mentioned drawbacksin the art can be overcome with the particular pump of the presentinvention which allows for preparation of homogeneous polyolefinproducts in particular polyethylene products that meet high qualitystandards from the complicated ethylene polymerization mixtures while atthe same time being produced at low energy consumption.

Accordingly, the present invention in particular relates to the use ofan axial pump for circulating an olefin polymerization slurry,preferably ethylene polymerization slurry comprising olefin monomer,preferably ethylene monomer, diluent and a polymerization catalystthrough a loop reactor for the production of polyolefin preferablypolyethylene, wherein said pump comprises an impeller consisting of 6blades and wherein said pump is fixed on a spring supported frame.

In another embodiment, the present invention relates to a method forproducing polyolefin in a loop reactor, comprising the steps of:

-   (a) feeding olefin monomer such as ethylene monomer, a diluent, and    at least one polymerization catalyst to said loop reactor to produce    a polymerization slurry;-   (b) polymerizing said ethylene in said loop reactor to produce a    polyolefin such as polyethylene,    wherein said slurry is circulated through said loop reactor by means    of an axial pump comprising an impeller consisting of 6 blades and    wherein said pump is fixed on a spring supported frame.

In another embodiment, the present invention relates to a loop reactorfor the production of polyolefin such as polyethylene, wherein saidreactor comprises

-   -   a plurality of interconnected pipes defining a flow path for a        polymerization slurry, said polymerization slurry comprising        olefin monomer such as ethylene monomer, optionally one or more        co-monomer(s), a polymerization catalyst, optionally an        activating agent, and diluent;    -   means for feeding monomer and diluent in the reactor;    -   means for feeding polymerization catalyst in the reactor;    -   optionally means for feeding an activating agent in the reactor;        and    -   a pump suitable for maintaining the polymer slurry in        circulation in said reactor,        wherein said pump is an axial pump with an impeller consisting        of 6 blades and wherein said pump is fixed on a spring supported        frame.

The present invention will now be further described. In the followingpassages, different aspects of the invention are defined in more detail.Each aspect so defined may be combined with any other aspect or aspectsunless clearly indicated to the contrary. In particular, any featureindicated as being preferred or advantageous may be combined with anyother feature or features indicated as being preferred or advantageous.The description is only given by way of example and does not limit theinvention. The reference numbers relate to the hereto-annexed figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates the details of a pump with a springsupported frame according to an embodiment of the present invention.

FIG. 2 schematically illustrates a double loop reactor with twoserially-connected loop reactors with a pump according to an embodimentof the present invention.

FIG. 3 represents a graph showing energy consumption of a four bladed(left side) and a pump according to an embodiment of the invention(right side).

DETAILED DESCRIPTION OF THE INVENTION

Before the present method and products of the invention are described,it is to be understood that this invention is not limited to particularmethods, components, products or combinations described, as suchmethods, components, products and combinations may, of course, vary. Itis also to be understood that the terminology used herein is notintended to be limiting, since the scope of the present invention willbe limited only by the appended claims.

As used herein, the singular forms “a”, “an”, and “the” include bothsingular and plural referents unless the context clearly dictatesotherwise.

The terms “comprising”, “comprises” and “comprised of” as used hereinare synonymous with “including”, “includes” or “containing”, “contains”,and are inclusive or open-ended and do not exclude additional,non-recited members, elements or method steps. It will be appreciatedthat the terms “comprising”, “comprises” and “comprised of” as usedherein comprise the terms “consisting of”, “consists” and “consists of”.

The recitation of numerical ranges by endpoints includes all numbers andfractions subsumed within the respective ranges, as well as the recitedendpoints.

The term “about” or “approximately” as used herein when referring to ameasurable value such as a parameter, an amount, a temporal duration,and the like, is meant to encompass variations of +1-10% or less,preferably +/−5% or less, more preferably +/−1% or less, and still morepreferably +/−0.1% or less of and from the specified value, insofar suchvariations are appropriate to perform in the disclosed invention. It isto be understood that the value to which the modifier “about” refers isitself also specifically, and preferably, disclosed.

All documents cited in the present specification are hereby incorporatedby reference in their entirety.

Unless otherwise defined, all terms used in disclosing the invention,including technical and scientific terms, have the meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. By means of further guidance, term definitions are included tobetter appreciate the teaching of the present invention.

In the following passages, different aspects of the invention aredefined in more detail. Each aspect so defined may be combined with anyother aspect or aspects unless clearly indicated to the contrary. Inparticular, any feature indicated as being preferred or advantageous maybe combined with any other feature or features indicated as beingpreferred or advantageous.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to a person skilled in the art from this disclosure, in one ormore embodiments. Furthermore, while some embodiments described hereininclude some but not other features included in other embodiments,combinations of features of different embodiments are meant to be withinthe scope of the invention, and form different embodiments, as would beunderstood by those in the art. For example, in the following claims,any of the claimed embodiments can be used in any combination.

According to the invention, an axial pump is used for circulating anolefin polymerization slurry, preferably an ethylene polymerizationslurry comprising ethylene monomer, diluent and a polymerizationcatalyst through a loop reactor for the production of polyolefin suchpreferably polyethylene, wherein said pump comprises an impellerconsisting of 6 blades and wherein said pump is fixed on a springsupported frame. In an embodiment said blades are made of inox.

As used herein, the term “axial pump” or “axial flow pump” refers to aimpeller (or propeller) circulation pump comprising an impeller that isfixed onto a rotor shaft which is directly or indirectly (e.g. viamagnetic coupling) driven by a motor, preferably an electromotor. Axialpumps allow fluid to enter the impeller axially and they discharge fluidnearly axially, circulating the liquid in a direction that is parallelto the rotor shaft.

According to an embodiment said impeller is fixed to a rotor shaft whichis attached to a motor. According to a preferred embodiment of theinvention, the impeller is fixed on one end of the rotor shaft insidethe tube of a loop polymerization reactor. Preferably, the rotor shaftextends outwards, through the wall of the tube of the looppolymerization reactor, where it is connected to the motor at the otherend of the shaft. Preferably, the shaft consists of a straight rod,preferably with rigid and direct orientation. Preferably, the pump shaftis at least partially surrounded by a sleeve. Preferably, the sleeve isconnected to the reactor. Preferably, the sleeve is connected with theinner wall of the reactor on the end of the impeller. Preferably, thesleeve is connected with the reactor wall at the position where theshaft extends outwards through the reactor wall. Preferably, the shaftand sleeve ensure optimal alignment of the motor, the impeller and thereactor, and prevent relative movement between the motor, the impellerand the reactor.

As used herein, the term “blade” refers to a vane or airfoil of theimpeller. A blade is a plate attached radially to a rotating drum,cylinder, or shaft. According to the invention, at least 5 blades,preferably 6 blades or more, are attached to the rotating shaft (i.e.the rotor shaft or impeller shaft) to collectively form the impeller,which moves a fluid by rotating. The blades can be fixed directly on theshaft or they can alternatively be fixed on a hub, which is positionedabout the shaft. The impeller, shaft and motor are the components of thepump.

In a preferred embodiment, the blades of said impeller have a bladeangle which is between 24° and 26°, more preferably between 24.5° and25.5°, and most preferably about 25°. As used herein, the term “bladeangle” refers to the angle which the chord line of the blade makes withthe impeller's rotational plane (in the direction of the rotation) andis expressed in degrees, whereby the chord line is a straight lineconnecting the leading and trailing edges of the blade, whereby theleading and trailing edge of a blade are respectively the front and backof the blade in the direction of motion. Blade angle is also referred toas angle of attack or pitch angle.

Preferably, the pump of the invention operates at a speed, or theoperational speed during the polymerization reaction or process, isbetween 1450 rpm (rotations per minute) and 1520 rpm, more preferablybetween 1470 rpm and 1500 rpm, and most preferably between 1480 rpm and1490 rpm, for instance 1485 rpm.

The diameter of the impeller is preferably larger than 70% and smallerthan 100% of the tube inner diameter of said loop reactor, morepreferably larger than 80% and smaller than 100%, and most preferablylarger than 90% and smaller than 100%. As used herein, the term “tubeinner diameter” means the diameter of the tube which is bordered by theinner surface of the tube. It is the total tube diameter (or thedistance between the opposing outer wall sections of the tube) minus twotimes the thickness of the wall of the tube.

Preferably, the impeller of said pump is located inside the tube of aloop reactor, more preferably just before a bend of the tube, i.e.upstream of a bend, relative to the flow direction.

Preferably, the blades and/or hub, and/or shaft are made from inox,alternatively referred to as stainless steel or corrosion-resistantsteel.

According to the invention the pump is fixed on a spring supportedframe. In an embodiment, said spring supported frame in firmly connectedto the ground. Preferably, the spring supported frame comprises one ormore springs, one or more frames, one or more connections to the reactorand one or more connections to the pump (preferably to the motor of thepump). Preferably, the connection to the reactor and the pump are rigid.Preferably, the springs are located below the frame. Preferably, thesprings adjustably support the frame allowing for alignment with theunderlying structure, for instance a floor. Preferably, the springsupported frame aligns the pump with the reactor path under varyingcircumstances, for instance the weight of the reactor, the weight of thepump and/or the changing temperature of the reactor or pump and/orprevent a change in physical orientation.

The present invention relates to the production of polyethylene. As usedherein, the term “particulate polyethylene” refers to polyethyleneparticles or granules.

Preferably the present invention concerns the production of monomodal,or multimodal such as bimodal polyethylene. As used herein, “multimodalpolyethylene” and “bimodal polyethylene” means polyethylene thatconsists of different polyethylene fractions, with differentphysicochemical and/or mechanical properties. By means of example, andwithout limitation, a bimodal polyethylene composition may comprise twopolyethylene fractions with a different average or mean molecularweight, melt flow index and/or density.

Preferably, said polyethylene has a median particle diameter (d50)comprised between 10 and 400 μm, more preferably between 50 and 300 μm,and most preferably around 200 μm. The d50 is generally measured bylaser diffraction analysis on a Malvern type analyser after having putthe particles in suspension in a solvent such as e.g. cyclohexane and isdefined as the particle size for which fifty percent by volume of theparticles has a size lower than the d50.

In an embodiment, the density of said polyethylene is between 0.900g/cm³ and 0.975 g/cm³, more preferably between 0.925 g/cm³ and 0.950g/cm³.

Polyethylene is produced from an ethylene polymerization slurry. As usedherein, the term “ethylene polymerization slurry” refers to acomposition comprising ethylene monomer, catalyst solid particles, i.e.a solid or particulate catalyst, and a diluent. The solid particles canbe suspended in the diluent. In the present invention it is especiallyapplicable to solid particles of ethylene polymerization catalyst in adiluent. Hence, in an embodiment, the ethylene polymerization slurryrefers to a mixture for homo-polymerization of ethylene monomer intopolyethylene.

In an embodiment, said ethylene polymerization slurry further comprisesone or more co-monomers. As used herein the term “co-monomer” refers toco-monomers which are suitable for being polymerized with ethylenemonomers. Co-monomers may comprise but are not limited to aliphaticC3-C20 alpha-olefins. Examples of suitable aliphatic C3-C20alpha-olefins include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene,1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene,1-octadecene and 1-eicosene. Hence, in an embodiment, the ethylenepolymerization slurry refers to a mixture for co-polymerization ofethylene monomer and co-monomer.

Diluents which are suitable for being used in accordance with thepresent invention are preferably liquid diluents which may comprise butare not limited to hydrocarbon diluents such as aliphatic,cycloaliphatic and aromatic hydrocarbon solvents, or halogenatedversions of such solvents. The preferred diluent are C12 or lower,straight chain or branched chain, saturated hydrocarbons, C5 to C9saturated alicyclic or aromatic hydrocarbons or C2 to C6 halogenatedhydrocarbons. Nonlimiting illustrative examples of diluents are butane,isobutane, pentane, hexane, heptane, cyclopentane, cyclohexane,cycloheptane, methyl cyclopentane, methyl cyclohexane, isooctane,benzene, toluene, xylene, chloroform, chlorobenzenes,tetrachloroethylene, dichloroethane and trichloroethane. In a preferredembodiment of the present invention, said diluent is isobutane. However,it should be clear from the present invention that other diluents may aswell be applied according to the present invention.

Preferably, the polymerization catalyst comprises a catalyst selectedfrom the group consisting of metallocene catalysts, Ziegler-Mattacatalysts and chromium catalysts, said catalyst being immobilized onto asolid inert carrier. By the term “solid particles” it is meant a solidprovided as a collection of particles, such as for instance a powder orgranulate. In the present invention it is especially applicable to acatalyst provided on a carrier or support. The support is preferably asilica (Si) support. The polymerization catalyst preferably has a meanparticle diameter) d50) comprised between 1 μm and 100 μm, morepreferably between 5 μm and 50 μm, and most preferably about 40 μm.

As used herein, the “catalyst” refers to a substance that causes achange in the rate of a polymerization reaction without itself beingconsumed in the reaction. In the present invention it is especiallyapplicable to catalysts suitable for the polymerization of ethylene topolyethylene. These catalysts will be referred to as ethylenepolymerization catalysts or polymerization catalysts. In the presentinvention it is especially applicable to ethylene polymerizationcatalysts such as metallocene catalysts, chromium catalysts and/orZiegler-Natta catalysts. Preferably, said polymerization catalystcomprises a catalyst selected from the group consisting of metallocenecatalysts, Ziegler-Natta catalysts and chromium catalysts, said catalystbeing immobilized onto a solid inert carrier

In a preferred embodiment of the present invention, said catalyst is ametallocene catalyst. The term “metallocene catalyst” is used herein todescribe any transition metal complexes consisting of metal atoms bondedto one or more ligands. The metallocene catalysts are compounds of GroupIV transition metals of the Periodic Table such as titanium, zirconium,hafnium, etc., and have a coordinated structure with a metal compoundand ligands composed of one or two groups of cyclopentadienyl, indenyl,fluorenyl or their derivatives. Use of metallocene catalysts in thepolymerization of polyethylene has various advantages. Metallocenecatalysts have high activities and are capable of preparing polymerswith enhanced physical properties. The key to metallocenes is thestructure of the complex. The structure and geometry of the metallocenecan be varied to adapt to the specific need of the producer depending onthe desired polymer. Metallocenes comprise a single metal site, whichallows for more control of branching and molecular weight distributionof the polymer. Monomers are inserted between the metal and the growingchain of polymer.

In a preferred embodiment, the metallocene catalyst has a generalformula (I) or (Il):(Ar)₂MQ₂   (I); orR″(Ar)₂MQ₂   (II)wherein the metallocenes according to formula (I) are non-bridgedmetallocenes and the metallocenes according to formula (II) are bridgedmetallocenes;

wherein said metallocene according to formula (I) or (II) has two Arbound to M which can be the same or different from each other;

wherein Ar is an aromatic ring, group or moiety and wherein each Ar isindependently selected from the group consisting of cyclopentadienyl,indenyl, tetrahydroindenyl or fluorenyl, wherein each of said groups maybe optionally substituted with one or more substituents eachindependently selected from the group consisting of halogen, ahydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and whereinsaid hydrocarbyl optionally contains one or more atoms selected from thegroup comprising B, Si, S, O, F, Cl and P;

wherein M is a transition metal M selected from the group consisting oftitanium, zirconium, hafnium and vanadium; and preferably is zirconium;

wherein each Q is independently selected from the group consisting ofhalogen; a hydrocarboxy having 1 to 20 carbon atoms; and a hydrocarbylhaving 1 to 20 carbon atoms and wherein said hydrocarbyl optionallycontains one or more atoms selected from the group comprising B, Si, S,O, F, Cl and P; and

wherein R″ is a divalent group or moiety bridging the two Ar groups andselected from the group consisting of a C₁-C₂₀ alkylene, a germanium, asilicon, a siloxane, an alkylphosphine and an amine, and wherein said R″is optionally substituted with one or more substituents eachindependently selected from the group consisting of halogen, ahydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and whereinsaid hydrocarbyl optionally contains one or more atoms selected from thegroup comprising B, Si, S, O, F, Cl and P.

The term “hydrocarbyl having 1 to 20 carbon atoms” as used herein isintended to refer to a moiety selected from the group comprising alinear or branched C₁-C₂₀ alkyl; C₃-C₂₀ cycloalkyl; C₆-C₂₀ aryl; C₇-C₂₀alkylaryl and C₇-C₂₀ arylalkyl, or any combinations thereof. Exemplaryhydrocarbyl groups are methyl, ethyl, propyl, butyl, amyl, isoamyl,hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, 2-ethylhexyl, andphenyl. Exemplary halogen atoms include chlorine, bromine, fluorine andiodine and of these halogen atoms, fluorine and chlorine are preferred.

Illustrative examples of metallocene catalysts comprise but are notlimited to bis(cyclopentadienyl)zirconium dichloride(Cp₂ZrCl₂),bis(cyclopentadienyl)titanium dichloride(Cp₂TiCl₂),bis(cyclopentadienyl)hafnium dichloride(Cp₂HfCl₂);bis(tetrahydroindenyl)zirconium dichloride, bis(indenyl)zirconiumdichloride, and bis(n-butyl-cyclopentadienyl)zirconium dichloride;ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride,ethylenebis(1-indenyl)zirconium dichloride, dimethylsilylenebis(2-methyl-4-phenyl-inden-1-yl)zirconium dichloride,diphenylmethylene(cyclopentadienyl)(fluoren-9-yl)zirconium dichloride,and dimethylmethylene[1-(4-tert-butyl-2-methyl-cyclopentadienyl)](fluoren-9-yl)zirconiumdichloride.

The catalysts can be provided on a solid support. The support should bean inert solid, which is chemically unreactive with any of thecomponents of the conventional metallocene catalyst. The support orcarrier is an inert organic or inorganic solid, which is chemicallyunreactive with any of the components of the conventional metallocenecatalyst. Suitable support materials for the supported catalyst of thepresent invention include solid inorganic oxides, such as silica,alumina, magnesium oxide, titanium oxide, thorium oxide, as well asmixed oxides of silica and one or more Group 2 or 13 metal oxides, suchas silica-magnesia and silica-alumina mixed oxides. Silica, alumina, andmixed oxides of silica and one or more Group 2 or 13 metal oxides arepreferred support materials. Preferred examples of such mixed oxides arethe silica-aluminas. Most preferred is silica. The silica may be ingranular, agglomerated, fumed or other form. The support is preferably asilica compound. In a preferred embodiment, the metallocene catalyst isprovided on a solid support, preferably a silica support.

In a preferred embodiment, a polymerization catalyst applied in thepresent polymerisation process is a supported metallocene-alumoxanecatalyst consisting of a metallocene and an alumoxane which are bound ona porous silica support.

In another embodiment of the present invention, said catalyst is achromium catalyst. The term “chromium catalysts” refers to catalystsobtained by deposition of chromium oxide on a support, e.g. a silica oraluminum support. Illustrative examples of chromium catalysts comprisebut are not limited to CrSiO₂ or CrAl2O₃.

In another embodiment of the present invention, said catalyst is aZiegler-Natta catalyst. The term “Ziegler-Natta catalyst” or “ZNcatalyst” refers to catalysts having a general formula M¹X_(v), whereinM¹ is a transition metal compound selected from group IV to VII, whereinX is a halogen, and wherein v is the valence of the metal. Preferably,M¹ is a group IV, group V or group VI metal, more preferably titanium,chromium or vanadium and most preferably titanium. Preferably, X ischlorine or bromine, and most preferably, chlorine. Illustrativeexamples of the transition metal compounds comprise but are not limitedto TiCl₃, TiCl₄. Suitable ZN catalysts for use in the invention aredescribed in U.S. Pat. Nos. 6,930,071 and 6,864,207, which areincorporated herein by reference.

In another embodiment, the present invention relates to a method forproducing polyolefin such aspolyethylene in a loop reactor, comprisingthe steps of:

-   -   (a) feeding olefin monomer such as ethylene monomer, a diluent,        and at least one polymerization catalyst to said loop reactor to        produce a polymerization slurry;    -   (b) polymerizing said ethylene in said loop reactor to produce        polyolefin such as polyethylene;        wherein said slurry is circulated through said loop reactor by        means of an axial pump comprising an impeller consisting of 6        blades, wherein said pump is fixed to a spring supported frame.

In a preferred embodiment, additionally one or more co-monomer is fed tothe loop reactor for the production of polyethylene co-polymers. Inanother preferred embodiment, additionally hydrogen is fed to the loopreactor.

Preferably, the polymerization slurry is circulated at a flow rate (i.e.the volumetric flow rate or rate of fluid flow) of from 4500 m³/h to6000 m³/h, more preferably from 5000 m³/h to 5500 m³/h, and mostpreferably about 5250 m³/h.

In a further embodiment, the loop reactor has a pressure of from 35 barg(equivalent to 36 bar, wherein barg is defined as bar+1) to 65 barg(equivalent to 66 bar), more preferably from 40 barg to 50 barg, mostpreferably from 40 barg and 45 barg, for example around 42 barg. In afurther embodiment, the pressure in said loop reactor during operationis at least 40 barg. As used herein, the term “berg” refers to “bargauge” or “gauge pressure”. Barg is zero referenced against ambient airpressure, so it is equal to absolute pressure minus atmosphericpressure. 1 barg is equivalent to 2 bar.

A further aspect of the invention relates to a loop reactor for theproduction of polyolefin such as polyethylene, wherein said reactorcomprises an axial pump with an impeller comprising at least 5 blades,preferably 6 blades or more, for circulating ethylene polymerizationslurry, wherein said pump is fixed to a spring supported frame. In apreferred embodiment, said blades are in inox.

As used herein, the term “loop reactor” refers to a closed circuittubular polymerization reactor for the production of polyethylene. Thesereactors consist of a long pipe, arranged in two loops. Loop reactorsare generally known in the art. The loop reactors as described hereinare liquid full reactors and are—essentially—free of a gaseous phasewhile in operation.

Specifically, the invention relates to a loop reactor for the productionof polyolefin such as polyethylene, wherein said reactor comprises

-   -   a plurality of interconnected pipes defining a flow path for a        polymerization slurry, said polymerization slurry comprising        ethylene monomer, optionally one or more co-monomer(s), a        polymerization catalyst, optionally an activating agent, and        diluent;    -   means for feeding monomer and diluent in the reactor;    -   means for feeding polymerization catalyst in the reactor;    -   optionally means for feeding an activating agent in the reactor;        and    -   a pump suitable for maintaining the polymer slurry in        circulation in said reactor,    -   wherein said pump is an axial pump with an impeller consisting        of 6 blades and wherein said pump is fixed on a spring supported        frame.

In an embodiment, said loop reactor has a tube diameter between 0.3 and0.7 meter, preferably between about 0.4 and 0.6 meter, more preferablybetween 0.45 m and 0.55 meter, and most preferably around 0.5 meter.

Preferably, the loop reactor has a linear path between 80 and 140 meter,more preferably between 95 and 125 meter, and most preferably about 110meter. As used herein, the term “linear path” or “linear flow path”means the total length of the tube of the loop reactor measured alongthe central axis of the tube. As such, it refers to the distance that afluid, in particular a polymerization slurry, travels, when circulated,between two consecutive passages of any given point in the reactor.

FIG. 1 schematically illustrates part of a polymerization reactor with apump and a spring supported frame according to an embodiment of thepresent invention. Pump 101 comprises a motor 102 connected with a shaftto six bladed impeller 103 which is located in reactor pipe 104. Springsupported frame 122 comprises springs 122-1, a frame 122-2, a reactorconnection 122-3 and a motor connection 122-4. Springs 122-1 are locatedbelow frame 122-2 and are connected to the bottom of frame 122-2 and tothe floor.

FIG. 2 shows a preferred polymerization reactor according to anembodiment of the present invention with two single loop reactors 100,116, which are interconnected in series. Both reactors 100, 116 consistof a plurality of interconnected pipes 104. The vertical sections of thepipe segments 104 are preferably provided with heat jackets 105.Reactants are introduced into the reactors 100 by line 107. The catalystand optionally the activation agent, may be fed in one or both reactors100 and 116 by means of conduct 106. The polymerization slurry isdirectionally circulated throughout the loop reactors 100, 116 asillustrated by the arrows 108 in the reactor path by one or more pumps,such as axial flow pump 101. The pump 101 may be powered by an electricmotor 102. The pump 101 may be provided with a set of rotating impellers103. Spring supported frame 122 comprises springs 122-1, a frame 122-2,a reactor connection 122-3 and a motor connection 122-4. Springs 122-1are located below frame 122-2 and are connected to the bottom of frame122-2. The reactors 100, 116 are further provided with one or moresettling legs 109 connected to the pipes 104 of the reactors 100, 116.The settling legs 109 are preferably provided with an isolation valve110. Further, the settling legs can be provided with product take off ordischarge valves 111 or can be in direct communication with thedownstream section. Downstream, the exit of the settling leg 109 ofreactor 100, a transfer line 112 is provided that allows for transfer ofthe polymer slurry settled in the settling legs 109 to the other reactor116 through a piston valve 115. Along transfer line 112, a three-wayvalve 114 may divert the flow to a product recovery zone if the multipleloop reactor has to be used in a parallel configuration. Polymer slurrysettled in the settling legs 109 of reactor 116 can be removed by meansof one or more product recovery lines 113, e.g. to a product recoveryzone.

The invention will now be illustrated by the following non-limitingexample.

EXAMPLE

In this example two serially connected reactors for example asillustrated in FIG. 2 were fitted with two standard pumps each having afour-bladed aluminum impeller with spring supported frame. Bimodalpolyethylene was prepared in the double loop reactor in the presence ofa particulate metallocene catalyst with a particle size of around 40 μm.

Subsequently, the reactor was shut down and the pumps in both reactorswere replaced with pumps having six bladed impellers and a springsupported frame according to an embodiment of the invention. The pumpswere set at 1485 rpm, the blade angle was about 25°, the size of theblade was approximately 95% of the tube inner diameter and the flow rateof the slurry was about 5250 m³/h. In both experiments, the pressure inthe reactor was kept at 42 barg, the reactor path length wasapproximately 110 meter, the tube inner diameter was approximately 0.5meter. Bimodal polyethylene was prepared in the double loop reactor inthe presence of a particulate polymerization catalyst with a particlesize of around 40 μm.

Surprisingly, while the produced polyethylene was of comparable highquality with a density of approximately 0.9235 g/cm³, it was found thatreplacement of the four-bladed pump by the six-bladed pump according tothe invention to circulate this complex slurry mixture, resulted inapproximately a 20% lower energy consumption in comparison to thepolymerization reaction wherein the standard 4-bladed pumps were used.FIG. 3 shows the energy consumption (in kW on the Y-axis) in time (inmonths on the X-axis) before (left side) and after (right side)installation of the six-bladed impeller according to the invention. Thediagonal lines indicate reactor shut down.

After replacement, a 20% reduction (on average) in energy consumptionwas observed. The top line is the energy consumption of the pump in thefirst reactor. The bottom line is the energy consumption of the pump inthe second reactor. It will be understood that the energy consumption ofthe pump is different in both reactors due to the different conditionsin each. It can be concluded that the present invention allows forpreparation of polyethylene of high quality from complex slurry mixtureswhile leading to lower energy consumption.

What is claimed:
 1. A method comprising: circulating an olefinpolymerization slurry through a loop reactor using an axial pump for theproduction of polyolefin, wherein the olefin polymerization slurrycomprises olefin monomer, diluent and a polymerization catalyst, whereinsaid axial pump comprises an impeller consisting of 6 blades, andwherein said axial pump is fixed on a spring supported frame.
 2. Themethod according to claim 1, wherein said blades have a blade angleranging between 24° and 26°.
 3. The method according to claim 1, whereinsaid blades are made of inox.
 4. The method according to claim 1,wherein said polyolefin is polyethylene.
 5. A method for producingpolyolefin in a loop reactor, comprising the steps of: (a) feedingolefin monomer, a diluent, and at least one polymerization catalyst tosaid loop reactor to produce a polymerization slurry; (b) polymerizingsaid olefin monomer in said loop reactor to produce a polyolefin,wherein said polymerization slurry is circulated through said loopreactor by means of an axial pump comprising an impeller consisting of 6blades, and wherein said axial pump is fixed on a spring supportedframe.
 6. The method according to claim 5, wherein said blades have ablade angle ranging between 24° and 26°.
 7. The method according toclaim 5, wherein said blades are made of inox.
 8. The method accordingto claim 5, wherein said polyolefin is polyethylene.
 9. A loop reactorfor the production of polyolefin, wherein said loop reactor comprises: aplurality of interconnected pipes defining a flow path for apolymerization slurry, said polymerization slurry comprising olefinmonomer, optionally one or more co-monomer(s), a polymerizationcatalyst, optionally an activating agent, and diluent; means for feedingmonomer and diluent in the loop reactor; means for feedingpolymerization catalyst in the reactor; optionally means for feeding anactivating agent in the reactor; and a pump suitable for maintaining thepolymerization slurry in circulation in said loop reactor, wherein saidpump is an axial pump with an impeller consisting of 6 blades, andwherein said pump is fixed on a spring supported frame.
 10. The loopreactor according to claim 9, wherein said blades have a blade angleranging between 24° and 26°.
 11. The loop reactor according to claim 9,wherein said blades are made of inox.
 12. The loop reactor according toclaim 9, wherein said polyolefin comprises polyethylene.